/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.*;

/**
 * An {@link ExecutorService} that executes each submitted task using one of possibly several pooled threads, normally
 * configured using {@link Executors} factory methods.
 *
 * <p>Thread pools address two different problems: they usually
 * provide improved performance when executing large numbers of asynchronous tasks, due to reduced per-task invocation
 * overhead, and they provide a means of bounding and managing the resources, including threads, consumed when executing
 * a collection of tasks. Each {@code ThreadPoolExecutor} also maintains some basic statistics, such as the number of
 * completed tasks.
 *
 * <p>To be useful across a wide range of contexts, this class
 * provides many adjustable parameters and extensibility hooks. However, programmers are urged to use the more
 * convenient {@link Executors} factory methods {@link Executors#newCachedThreadPool} (unbounded thread pool, with
 * automatic thread reclamation), {@link Executors#newFixedThreadPool} (fixed size thread pool) and {@link
 * Executors#newSingleThreadExecutor} (single background thread), that preconfigure settings for the most common usage
 * scenarios. Otherwise, use the following guide when manually configuring and tuning this class:
 *
 * <dl>
 *
 * <dt>Core and maximum pool sizes</dt>
 *
 * <dd>A {@code ThreadPoolExecutor} will automatically adjust the
 * pool size (see {@link #getPoolSize})
 * according to the bounds set by
 * corePoolSize (see {@link #getCorePoolSize}) and
 * maximumPoolSize (see {@link #getMaximumPoolSize}).
 * <p>
 * When a new task is submitted in method {@link #execute(Runnable)},
 * and fewer than corePoolSize threads are running, a new thread is
 * created to handle the request, even if other worker threads are
 * idle.  If there are more than corePoolSize but less than
 * maximumPoolSize threads running, a new thread will be created only
 * if the queue is full.  By setting corePoolSize and maximumPoolSize
 * the same, you create a fixed-size thread pool. By setting
 * maximumPoolSize to an essentially unbounded value such as {@code
 * Integer.MAX_VALUE}, you allow the pool to accommodate an arbitrary
 * number of concurrent tasks. Most typically, core and maximum pool
 * sizes are set only upon construction, but they may also be changed
 * dynamically using {@link #setCorePoolSize} and {@link
 * #setMaximumPoolSize}. </dd>
 *
 * <dt>On-demand construction</dt>
 *
 * <dd>By default, even core threads are initially created and
 * started only when new tasks arrive, but this can be overridden
 * dynamically using method {@link #prestartCoreThread} or {@link
 * #prestartAllCoreThreads}.  You probably want to prestart threads if
 * you construct the pool with a non-empty queue. </dd>
 *
 * <dt>Creating new threads</dt>
 *
 * <dd>New threads are created using a {@link ThreadFactory}.  If not
 * otherwise specified, a {@link Executors#defaultThreadFactory} is
 * used, that creates threads to all be in the same {@link
 * ThreadGroup} and with the same {@code NORM_PRIORITY} priority and
 * non-daemon status. By supplying a different ThreadFactory, you can
 * alter the thread's name, thread group, priority, daemon status,
 * etc. If a {@code ThreadFactory} fails to create a thread when asked
 * by returning null from {@code newThread}, the executor will
 * continue, but might not be able to execute any tasks. Threads
 * should possess the "modifyThread" {@code RuntimePermission}. If
 * worker threads or other threads using the pool do not possess this
 * permission, service may be degraded: configuration changes may not
 * take effect in a timely manner, and a shutdown pool may remain in a
 * state in which termination is possible but not completed.</dd>
 *
 * <dt>Keep-alive times</dt>
 *
 * <dd>If the pool currently has more than corePoolSize threads,
 * excess threads will be terminated if they have been idle for more
 * than the keepAliveTime (see {@link #getKeepAliveTime(TimeUnit)}).
 * This provides a means of reducing resource consumption when the
 * pool is not being actively used. If the pool becomes more active
 * later, new threads will be constructed. This parameter can also be
 * changed dynamically using method {@link #setKeepAliveTime(long,
 * TimeUnit)}.  Using a value of {@code Long.MAX_VALUE} {@link
 * TimeUnit#NANOSECONDS} effectively disables idle threads from ever
 * terminating prior to shut down. By default, the keep-alive policy
 * applies only when there are more than corePoolSize threads. But
 * method {@link #allowCoreThreadTimeOut(boolean)} can be used to
 * apply this time-out policy to core threads as well, so long as the
 * keepAliveTime value is non-zero. </dd>
 *
 * <dt>Queuing</dt>
 *
 * <dd>Any {@link BlockingQueue} may be used to transfer and hold
 * submitted tasks.  The use of this queue interacts with pool sizing:
 *
 * <ul>
 *
 * <li> If fewer than corePoolSize threads are running, the Executor
 * always prefers adding a new thread
 * rather than queuing.</li>
 *
 * <li> If corePoolSize or more threads are running, the Executor
 * always prefers queuing a request rather than adding a new
 * thread.</li>
 *
 * <li> If a request cannot be queued, a new thread is created unless
 * this would exceed maximumPoolSize, in which case, the task will be
 * rejected.</li>
 *
 * </ul>
 * <p>
 * There are three general strategies for queuing:
 * <ol>
 *
 * <li> <em> Direct handoffs.</em> A good default choice for a work
 * queue is a {@link SynchronousQueue} that hands off tasks to threads
 * without otherwise holding them. Here, an attempt to queue a task
 * will fail if no threads are immediately available to run it, so a
 * new thread will be constructed. This policy avoids lockups when
 * handling sets of requests that might have internal dependencies.
 * Direct handoffs generally require unbounded maximumPoolSizes to
 * avoid rejection of new submitted tasks. This in turn admits the
 * possibility of unbounded thread growth when commands continue to
 * arrive on average faster than they can be processed.  </li>
 *
 * <li><em> Unbounded queues.</em> Using an unbounded queue (for
 * example a {@link LinkedBlockingQueue} without a predefined
 * capacity) will cause new tasks to wait in the queue when all
 * corePoolSize threads are busy. Thus, no more than corePoolSize
 * threads will ever be created. (And the value of the maximumPoolSize
 * therefore doesn't have any effect.)  This may be appropriate when
 * each task is completely independent of others, so tasks cannot
 * affect each others execution; for example, in a web page server.
 * While this style of queuing can be useful in smoothing out
 * transient bursts of requests, it admits the possibility of
 * unbounded work queue growth when commands continue to arrive on
 * average faster than they can be processed.  </li>
 *
 * <li><em>Bounded queues.</em> A bounded queue (for example, an
 * {@link ArrayBlockingQueue}) helps prevent resource exhaustion when
 * used with finite maximumPoolSizes, but can be more difficult to
 * tune and control.  Queue sizes and maximum pool sizes may be traded
 * off for each other: Using large queues and small pools minimizes
 * CPU usage, OS resources, and context-switching overhead, but can
 * lead to artificially low throughput.  If tasks frequently block (for
 * example if they are I/O bound), a system may be able to schedule
 * time for more threads than you otherwise allow. Use of small queues
 * generally requires larger pool sizes, which keeps CPUs busier but
 * may encounter unacceptable scheduling overhead, which also
 * decreases throughput.  </li>
 *
 * </ol>
 *
 * </dd>
 *
 * <dt>Rejected tasks</dt>
 *
 * <dd>New tasks submitted in method {@link #execute(Runnable)} will be
 * <em>rejected</em> when the Executor has been shut down, and also when
 * the Executor uses finite bounds for both maximum threads and work queue
 * capacity, and is saturated.  In either case, the {@code execute} method
 * invokes the {@link
 * RejectedExecutionHandler#rejectedExecution(Runnable, ThreadPoolExecutor)}
 * method of its {@link RejectedExecutionHandler}.  Four predefined handler
 * policies are provided:
 *
 * <ol>
 *
 * <li> In the default {@link ThreadPoolExecutor.AbortPolicy}, the
 * handler throws a runtime {@link RejectedExecutionException} upon
 * rejection. </li>
 *
 * <li> In {@link ThreadPoolExecutor.CallerRunsPolicy}, the thread
 * that invokes {@code execute} itself runs the task. This provides a
 * simple feedback control mechanism that will slow down the rate that
 * new tasks are submitted. </li>
 *
 * <li> In {@link ThreadPoolExecutor.DiscardPolicy}, a task that
 * cannot be executed is simply dropped.  </li>
 *
 * <li>In {@link ThreadPoolExecutor.DiscardOldestPolicy}, if the
 * executor is not shut down, the task at the head of the work queue
 * is dropped, and then execution is retried (which can fail again,
 * causing this to be repeated.) </li>
 *
 * </ol>
 * <p>
 * It is possible to define and use other kinds of {@link
 * RejectedExecutionHandler} classes. Doing so requires some care
 * especially when policies are designed to work only under particular
 * capacity or queuing policies. </dd>
 *
 * <dt>Hook methods</dt>
 *
 * <dd>This class provides {@code protected} overridable
 * {@link #beforeExecute(Thread, Runnable)} and
 * {@link #afterExecute(Runnable, Throwable)} methods that are called
 * before and after execution of each task.  These can be used to
 * manipulate the execution environment; for example, reinitializing
 * ThreadLocals, gathering statistics, or adding log entries.
 * Additionally, method {@link #terminated} can be overridden to perform
 * any special processing that needs to be done once the Executor has
 * fully terminated.
 *
 * <p>If hook or callback methods throw exceptions, internal worker
 * threads may in turn fail and abruptly terminate.</dd>
 *
 * <dt>Queue maintenance</dt>
 *
 * <dd>Method {@link #getQueue()} allows access to the work queue
 * for purposes of monitoring and debugging.  Use of this method for
 * any other purpose is strongly discouraged.  Two supplied methods,
 * {@link #remove(Runnable)} and {@link #purge} are available to
 * assist in storage reclamation when large numbers of queued tasks
 * become cancelled.</dd>
 *
 * <dt>Finalization</dt>
 *
 * <dd>A pool that is no longer referenced in a program <em>AND</em>
 * has no remaining threads will be {@code shutdown} automatically. If
 * you would like to ensure that unreferenced pools are reclaimed even
 * if users forget to call {@link #shutdown}, then you must arrange
 * that unused threads eventually die, by setting appropriate
 * keep-alive times, using a lower bound of zero core threads and/or
 * setting {@link #allowCoreThreadTimeOut(boolean)}.  </dd>
 *
 * </dl>
 *
 * <p><b>Extension example</b>. Most extensions of this class
 * override one or more of the protected hook methods. For example,
 * here is a subclass that adds a simple pause/resume feature:
 *
 *  <pre> {@code
 * class PausableThreadPoolExecutor extends ThreadPoolExecutor {
 *   private boolean isPaused;
 *   private ReentrantLock pauseLock = new ReentrantLock();
 *   private Condition unpaused = pauseLock.newCondition();
 *
 *   public PausableThreadPoolExecutor(...) { super(...); }
 *
 *   protected void beforeExecute(Thread t, Runnable r) {
 *     super.beforeExecute(t, r);
 *     pauseLock.lock();
 *     try {
 *       while (isPaused) unpaused.await();
 *     } catch (InterruptedException ie) {
 *       t.interrupt();
 *     } finally {
 *       pauseLock.unlock();
 *     }
 *   }
 *
 *   public void pause() {
 *     pauseLock.lock();
 *     try {
 *       isPaused = true;
 *     } finally {
 *       pauseLock.unlock();
 *     }
 *   }
 *
 *   public void resume() {
 *     pauseLock.lock();
 *     try {
 *       isPaused = false;
 *       unpaused.signalAll();
 *     } finally {
 *       pauseLock.unlock();
 *     }
 *   }
 * }}</pre>
 *
 * @author Doug Lea
 * @since 1.5
 */
public class ThreadPoolExecutor extends AbstractExecutorService {
    
    /**
     * The main pool control state, ctl, is an atomic integer packing two conceptual fields workerCount, indicating the
     * effective number of threads runState,    indicating whether running, shutting down etc
     * <p>
     * In order to pack them into one int, we limit workerCount to (2^29)-1 (about 500 million) threads rather than
     * (2^31)-1 (2 billion) otherwise representable. If this is ever an issue in the future, the variable can be changed
     * to be an AtomicLong, and the shift/mask constants below adjusted. But until the need arises, this code is a bit
     * faster and simpler using an int.
     * <p>
     * The workerCount is the number of workers that have been permitted to start and not permitted to stop.  The value
     * may be transiently different from the actual number of live threads, for example when a ThreadFactory fails to
     * create a thread when asked, and when exiting threads are still performing bookkeeping before terminating. The
     * user-visible pool size is reported as the current size of the workers set.
     * <p>
     * The runState provides the main lifecycle control, taking on values:
     * <p>
     * RUNNING:  Accept new tasks and process queued tasks SHUTDOWN: Don't accept new tasks, but process queued tasks
     * STOP:     Don't accept new tasks, don't process queued tasks, and interrupt in-progress tasks TIDYING:  All tasks
     * have terminated, workerCount is zero, the thread transitioning to state TIDYING will run the terminated() hook
     * method TERMINATED: terminated() has completed
     * <p>
     * The numerical order among these values matters, to allow ordered comparisons. The runState monotonically
     * increases over time, but need not hit each state. The transitions are:
     * <p>
     * RUNNING -> SHUTDOWN On invocation of shutdown(), perhaps implicitly in finalize() (RUNNING or SHUTDOWN) -> STOP
     * On invocation of shutdownNow() SHUTDOWN -> TIDYING When both queue and pool are empty STOP -> TIDYING When pool
     * is empty TIDYING -> TERMINATED When the terminated() hook method has completed
     * <p>
     * Threads waiting in awaitTermination() will return when the state reaches TERMINATED.
     * <p>
     * Detecting the transition from SHUTDOWN to TIDYING is less straightforward than you'd like because the queue may
     * become empty after non-empty and vice versa during SHUTDOWN state, but we can only terminate if, after seeing
     * that it is empty, we see that workerCount is 0 (which sometimes entails a recheck -- see below).
     */
    
    /**
     * 参考:
     * 1. https://fangjian0423.github.io/2016/03/22/java-threadpool-analysis/
     *
     * 线程池的 5 个状态
     * 1. running 可以接收新的任务, 也可以处理阻塞队列里的任务
     * 2. shutdown 不接受新的任务,但是可以处理阻塞队列里的任务
     * 3. stop 不接受新任务,不处理阻塞队列任务,中断正在处理的任务
     * 4. tidying 过渡状态,即 所有的任务都执行完,当前线程池已经没有有效的线程,这个时候线程池的状态改为 tidying,并将调用 terminated 方法
     * 5. terminated 终止状态,terminated 方法调用后的状态
     *
     * 状态转换:
     * running > shutdown: 手动调用 shutdown 方法,或者 ThreadPoolExecutor 要被 GC 回收的时候调用 ThreadPoolExecutor.finalize 方法
     *                      finalize 方法内部也会调用 shutdown 方法.
     * (running|shutdown) > shtop : 调用 shutdownNow 方法.
     * shutdown > tidying: 当队列和线程池都为空的时候,
     * stop > tidying: 当线程池为空的时候
     * tidying > terminated : terminated 方法调用完成后
     *
     * ThreadPoolExecutor 内部使用 ctl 存储着线程池的有效线程个数. 而且 ctl 还记录这线程的状态.
     * 所以是 一个变量标识 2 种含义.
     *
     * 线程池执行任务
     * 可以使用 execute 方法或者 submit 方法, 而 submit 方法是在 ExecutorService 接口定义的. 实现在
     * java.util.concurrent.AbstractExecutorService#submit(java.util.concurrent.Callable)
     * 不论有无返回值的线程 submit,内部都是调用的 execute 方法.
     * public <T> Future<T> submit(Callable<T> task) {
     *     if (task == null) throw new NullPointerException();
     *     RunnableFuture<T> ftask = newTaskFor(task);
     *     execute(ftask);
     *     return ftask;
     * }
     * 有返回值的是将 thread 用 future 包装一层.
     *
     * 我们只需要去看看 execute 的执行逻辑
     * java.util.concurrent.ThreadPoolExecutor#execute(java.lang.Runnable)
     */
    
    /**
     * 初始化线程池状态为 running, 线程数量为 0
     */
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    
    /**
     * 前 3 位表示状态,所有线程数占 29 位
     */
    private static final int COUNT_BITS = Integer.SIZE - 3;
    /**
     * 1 左移 1 位,=10,左移 2 位=100,所以 左移几位就说明后面跟着几个 0,
     * 线程池的容量大小为 1 << 29 - 1 = 0001 1111 1111 1111 1111 1111 1111 1111
     */
    private static final int CAPACITY = (1 << COUNT_BITS) - 1;
    
    // runState is stored in the high-order bits
    /**
     * running 状态 = -1 << 29 = 1111 1111 1111 1111 1111 1111 1111 1111 << 29 = 1110 0000 ... 后面 29 个 0
     * 这里是补码, 补码 = 正码取反 + 1
     * 也就是 running 状态为前 3 位为:111
     */
    private static final int RUNNING = -1 << COUNT_BITS;
    /**
     * shutdown 状态= 0 << 29, 前 3 位: 000
     */
    private static final int SHUTDOWN = 0 << COUNT_BITS;
    /**
     * stop: 前 3 位: 001
     */
    private static final int STOP = 1 << COUNT_BITS;
    /**
     * tidying: 010
     */
    private static final int TIDYING = 2 << COUNT_BITS;
    /**
     * terminated: 011
     */
    private static final int TERMINATED = 3 << COUNT_BITS;
    
    // Packing and unpacking ctl
    
    /**
     * 得到状态, 取 c 前 3 位,得到状态码
     * @param c
     * @return
     */
    private static int runStateOf(int c) {
        return c & ~CAPACITY;
    }
    
    /**
     * 得到线程数, 后 29 位
     * @param c
     * @return
     */
    private static int workerCountOf(int c) {
        return c & CAPACITY;
    }
    
    /**
     * 或操作, 更新 线程池状态 和 线程数量
     * @param rs
     * @param wc
     * @return
     */
    private static int ctlOf(int rs, int wc) {
        return rs | wc;
    }
    
    /*
     * Bit field accessors that don't require unpacking ctl.
     * These depend on the bit layout and on workerCount being never negative.
     */
    
    private static boolean runStateLessThan(int c, int s) {
        return c < s;
    }
    
    private static boolean runStateAtLeast(int c, int s) {
        return c >= s;
    }
    
    private static boolean isRunning(int c) {
        return c < SHUTDOWN;
    }
    
    /**
     * Attempts to CAS-increment the workerCount field of ctl.
     */
    private boolean compareAndIncrementWorkerCount(int expect) {
        return ctl.compareAndSet(expect, expect + 1);
    }
    
    /**
     * Attempts to CAS-decrement the workerCount field of ctl.
     */
    private boolean compareAndDecrementWorkerCount(int expect) {
        return ctl.compareAndSet(expect, expect - 1);
    }
    
    /**
     * Decrements the workerCount field of ctl. This is called only on abrupt termination of a thread (see
     * processWorkerExit). Other decrements are performed within getTask.
     */
    private void decrementWorkerCount() {
        do {
        } while (!compareAndDecrementWorkerCount(ctl.get()));
    }
    
    /**
     * The queue used for holding tasks and handing off to worker threads.  We do not require that workQueue.poll()
     * returning null necessarily means that workQueue.isEmpty(), so rely solely on isEmpty to see if the queue is empty
     * (which we must do for example when deciding whether to transition from SHUTDOWN to TIDYING).  This accommodates
     * special-purpose queues such as DelayQueues for which poll() is allowed to return null even if it may later return
     * non-null when delays expire.
     */
    private final BlockingQueue<Runnable> workQueue;
    
    /**
     * Lock held on access to workers set and related bookkeeping. While we could use a concurrent set of some sort, it
     * turns out to be generally preferable to use a lock. Among the reasons is that this serializes
     * interruptIdleWorkers, which avoids unnecessary interrupt storms, especially during shutdown. Otherwise exiting
     * threads would concurrently interrupt those that have not yet interrupted. It also simplifies some of the
     * associated statistics bookkeeping of largestPoolSize etc. We also hold mainLock on shutdown and shutdownNow, for
     * the sake of ensuring workers set is stable while separately checking permission to interrupt and actually
     * interrupting.
     */
    private final ReentrantLock mainLock = new ReentrantLock();
    
    /**
     * Set containing all worker threads in pool. Accessed only when holding mainLock.
     */
    private final HashSet<Worker> workers = new HashSet<Worker>();
    
    /**
     * Wait condition to support awaitTermination
     */
    private final Condition termination = mainLock.newCondition();
    
    /**
     * Tracks largest attained pool size. Accessed only under mainLock.
     */
    private int largestPoolSize;
    
    /**
     * Counter for completed tasks. Updated only on termination of worker threads. Accessed only under mainLock.
     */
    private long completedTaskCount;
    
    /*
     * All user control parameters are declared as volatiles so that
     * ongoing actions are based on freshest values, but without need
     * for locking, since no internal invariants depend on them
     * changing synchronously with respect to other actions.
     */
    
    /**
     * Factory for new threads. All threads are created using this factory (via method addWorker).  All callers must be
     * prepared for addWorker to fail, which may reflect a system or user's policy limiting the number of threads.  Even
     * though it is not treated as an error, failure to create threads may result in new tasks being rejected or
     * existing ones remaining stuck in the queue.
     * <p>
     * We go further and preserve pool invariants even in the face of errors such as OutOfMemoryError, that might be
     * thrown while trying to create threads.  Such errors are rather common due to the need to allocate a native stack
     * in Thread.start, and users will want to perform clean pool shutdown to clean up.  There will likely be enough
     * memory available for the cleanup code to complete without encountering yet another OutOfMemoryError.
     */
    private volatile ThreadFactory threadFactory;
    
    /**
     * Handler called when saturated or shutdown in execute.
     */
    private volatile RejectedExecutionHandler handler;
    
    /**
     * Timeout in nanoseconds for idle threads waiting for work. Threads use this timeout when there are more than
     * corePoolSize present or if allowCoreThreadTimeOut. Otherwise they wait forever for new work.
     */
    private volatile long keepAliveTime;
    
    /**
     * If false (default), core threads stay alive even when idle. If true, core threads use keepAliveTime to time out
     * waiting for work.
     */
    private volatile boolean allowCoreThreadTimeOut;
    
    /**
     * Core pool size is the minimum number of workers to keep alive (and not allow to time out etc) unless
     * allowCoreThreadTimeOut is set, in which case the minimum is zero.
     */
    private volatile int corePoolSize;
    
    /**
     * Maximum pool size. Note that the actual maximum is internally bounded by CAPACITY.
     */
    private volatile int maximumPoolSize;
    
    /**
     * The default rejected execution handler
     */
    private static final RejectedExecutionHandler defaultHandler =
            new AbortPolicy();
    
    /**
     * Permission required for callers of shutdown and shutdownNow. We additionally require (see checkShutdownAccess)
     * that callers have permission to actually interrupt threads in the worker set (as governed by Thread.interrupt,
     * which relies on ThreadGroup.checkAccess, which in turn relies on SecurityManager.checkAccess). Shutdowns are
     * attempted only if these checks pass.
     * <p>
     * All actual invocations of Thread.interrupt (see interruptIdleWorkers and interruptWorkers) ignore
     * SecurityExceptions, meaning that the attempted interrupts silently fail. In the case of shutdown, they should not
     * fail unless the SecurityManager has inconsistent policies, sometimes allowing access to a thread and sometimes
     * not. In such cases, failure to actually interrupt threads may disable or delay full termination. Other uses of
     * interruptIdleWorkers are advisory, and failure to actually interrupt will merely delay response to configuration
     * changes so is not handled exceptionally.
     */
    private static final RuntimePermission shutdownPerm =
            new RuntimePermission("modifyThread");
    
    /* The context to be used when executing the finalizer, or null. */
    private final AccessControlContext acc;
    
    /**
     * Class Worker mainly maintains interrupt control state for threads running tasks, along with other minor
     * bookkeeping. This class opportunistically extends AbstractQueuedSynchronizer to simplify acquiring and releasing
     * a lock surrounding each task execution.  This protects against interrupts that are intended to wake up a worker
     * thread waiting for a task from instead interrupting a task being run.  We implement a simple non-reentrant mutual
     * exclusion lock rather than use ReentrantLock because we do not want worker tasks to be able to reacquire the lock
     * when they invoke pool control methods like setCorePoolSize.  Additionally, to suppress interrupts until the
     * thread actually starts running tasks, we initialize lock state to a negative value, and clear it upon start (in
     * runWorker).
     *
     * 内部类, worker 是一个 AQS 的实现类(原子操作),实现了 runnable 类
     * worker 才是后面真正执行 task 的对象,会调用 worker.run()方法.
     * worker 持有一个 task 的引用.
     *
     */
    private final class Worker
            extends AbstractQueuedSynchronizer
            implements Runnable {
        
        /**
         * This class will never be serialized, but we provide a serialVersionUID to suppress a javac warning.
         */
        private static final long serialVersionUID = 6138294804551838833L;
        
        /**
         * Thread this worker is running in.  Null if factory fails.
         */
        final Thread thread;
        /**
         * Initial task to run.  Possibly null.
         */
        Runnable firstTask;
        /**
         * Per-thread task counter
         * 标识这个 worker 完成的任务数
         */
        volatile long completedTasks;
        
        /**
         * Creates with given first task and thread from ThreadFactory.
         *
         * @param firstTask the first task (null if none)
         */
        Worker(Runnable firstTask) {
            /**
             * 状态设置为 -1,这样任何线程都不能得到 worker 的锁,
             * 除非调用了 unlock 方法,这个 unlock 方法会在 runWorker 方法用一开始就调用
             * 为了确保 worker 构造出来后,没有任何线程能够得到它的锁,除非调用 runworker 之后,
             * 其他线程才能获得 worker 的锁.
             */
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            /**
             * getThreadFactory() 方法用来在线程池中创建新的线程去执行任务.
             * 如果我们使用的是 7 参数构造线程池,那么就是第 6 个参数,默认值是:Executors.defaultThreadFactory()
             * 得到了线程工厂, 然后 newThread(this) 传参是 this,表示工厂创建的就是这个 worker thread.
             * 在我们线程池执行线程使用 x.start() 的时候,就是调用的 worker.run()方法.再去看 worker.run
             * 里面调用了父类的 runWorker(this) 方法.
             * 而我们观察到这个方法内部得到了 worker 绑定的真实任务 task,然后调用了她的 run
             * Runnable task = w.firstTask;
             * task.run();
             *
             * 我们大概猜测到了,线程池执行任务是交给 worker 去启动的.
             *
             */
            this.thread = getThreadFactory().newThread(this);
        }
        
        /**
         * Delegates main run loop to outer runWorker
         */
        public void run() {
            runWorker(this);
        }
        
        // Lock methods
        //
        // The value 0 represents the unlocked state.
        // The value 1 represents the locked state.
        
        protected boolean isHeldExclusively() {
            return getState() != 0;
        }
        
        protected boolean tryAcquire(int unused) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }
        
        protected boolean tryRelease(int unused) {
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }
        
        public void lock() {
            acquire(1);
        }
        
        public boolean tryLock() {
            return tryAcquire(1);
        }
        
        public void unlock() {
            release(1);
        }
        
        public boolean isLocked() {
            return isHeldExclusively();
        }
        
        void interruptIfStarted() {
            Thread t;
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }
    }
    
    /*
     * Methods for setting control state
     */
    
    /**
     * Transitions runState to given target, or leaves it alone if already at least the given target.
     *
     * @param targetState the desired state, either SHUTDOWN or STOP (but not TIDYING or TERMINATED -- use tryTerminate
     *                    for that)
     */
    private void advanceRunState(int targetState) {
        for (; ; ) {
            int c = ctl.get();
            if (runStateAtLeast(c, targetState) ||
                    ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c)))) {
                break;
            }
        }
    }
    
    /**
     * Transitions to TERMINATED state if either (SHUTDOWN and pool and queue empty) or (STOP and pool empty).  If
     * otherwise eligible to terminate but workerCount is nonzero, interrupts an idle worker to ensure that shutdown
     * signals propagate. This method must be called following any action that might make termination possible --
     * reducing worker count or removing tasks from the queue during shutdown. The method is non-private to allow access
     * from ScheduledThreadPoolExecutor.
     */
    final void tryTerminate() {
        for (; ; ) {
            int c = ctl.get();
            /**
             * 满足 3 个条件的任意，就不终止
             * 1、线程池还在运行，running
             * 2、处于 tidying 或者 terminated，说明已经在关闭途中，不需要处理
             * 3、shutdown 状态且阻塞队列不为空，需要处理队列的任务，不能关闭线程池。
             */
            if (isRunning(c) ||
                    runStateAtLeast(c, TIDYING) ||
                    (runStateOf(c) == SHUTDOWN && !workQueue.isEmpty())) {
                return;
            }
            // 如果到这里，说明线程池已经不再运行，队列没有任务，但是还要回收 worker
            if (workerCountOf(c) != 0) { // Eligible to terminate
                /**
                 * 一个 worker 一个 worker 的中断，因为中断 worker 还会调用 tryTerminate，所以不需要一次性全部中断
                 */
                interruptIdleWorkers(ONLY_ONE);
                return;
            }
            // worker 全部回收了，准备结束线程池
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                    try {
                        terminated();
                    } finally {
                        // 状态变更
                        ctl.set(ctlOf(TERMINATED, 0));
                        termination.signalAll();
                    }
                    return;
                }
            } finally {
                mainLock.unlock();
            }
            // else retry on failed CAS
        }
    }
    
    /*
     * Methods for controlling interrupts to worker threads.
     */
    
    /**
     * If there is a security manager, makes sure caller has permission to shut down threads in general (see
     * shutdownPerm). If this passes, additionally makes sure the caller is allowed to interrupt each worker thread.
     * This might not be true even if first check passed, if the SecurityManager treats some threads specially.
     */
    private void checkShutdownAccess() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkPermission(shutdownPerm);
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                for (Worker w : workers) {
                    security.checkAccess(w.thread);
                }
            } finally {
                mainLock.unlock();
            }
        }
    }
    
    /**
     * Interrupts all threads, even if active. Ignores SecurityExceptions (in which case some threads may remain
     * uninterrupted).
     */
    private void interruptWorkers() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                w.interruptIfStarted();
            }
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Interrupts threads that might be waiting for tasks (as indicated by not being locked) so they can check for
     * termination or configuration changes. Ignores SecurityExceptions (in which case some threads may remain
     * uninterrupted).
     *
     * @param onlyOne If true, interrupt at most one worker. This is called only from tryTerminate when termination is
     *                otherwise enabled but there are still other workers.  In this case, at most one waiting worker is
     *                interrupted to propagate shutdown signals in case all threads are currently waiting. Interrupting
     *                any arbitrary thread ensures that newly arriving workers since shutdown began will also eventually
     *                exit. To guarantee eventual termination, it suffices to always interrupt only one idle worker, but
     *                shutdown() interrupts all idle workers so that redundant workers exit promptly, not waiting for a
     *                straggler task to finish.
     */
    private void interruptIdleWorkers(boolean onlyOne) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                Thread t = w.thread;
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                if (onlyOne) {
                    break;
                }
            }
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Common form of interruptIdleWorkers, to avoid having to remember what the boolean argument means.
     */
    private void interruptIdleWorkers() {
        interruptIdleWorkers(false);
    }
    
    private static final boolean ONLY_ONE = true;
    
    /*
     * Misc utilities, most of which are also exported to
     * ScheduledThreadPoolExecutor
     */
    
    /**
     * Invokes the rejected execution handler for the given command. Package-protected for use by
     * ScheduledThreadPoolExecutor.
     */
    final void reject(Runnable command) {
        handler.rejectedExecution(command, this);
    }
    
    /**
     * Performs any further cleanup following run state transition on invocation of shutdown.  A no-op here, but used by
     * ScheduledThreadPoolExecutor to cancel delayed tasks.
     */
    void onShutdown() {
    }
    
    /**
     * State check needed by ScheduledThreadPoolExecutor to enable running tasks during shutdown.
     *
     * @param shutdownOK true if should return true if SHUTDOWN
     */
    final boolean isRunningOrShutdown(boolean shutdownOK) {
        int rs = runStateOf(ctl.get());
        return rs == RUNNING || (rs == SHUTDOWN && shutdownOK);
    }
    
    /**
     * Drains the task queue into a new list, normally using drainTo. But if the queue is a DelayQueue or any other kind
     * of queue for which poll or drainTo may fail to remove some elements, it deletes them one by one.
     */
    private List<Runnable> drainQueue() {
        BlockingQueue<Runnable> q = workQueue;
        ArrayList<Runnable> taskList = new ArrayList<Runnable>();
        q.drainTo(taskList);
        if (!q.isEmpty()) {
            for (Runnable r : q.toArray(new Runnable[0])) {
                if (q.remove(r)) {
                    taskList.add(r);
                }
            }
        }
        return taskList;
    }
    
    /*
     * Methods for creating, running and cleaning up after workers
     */
    
    /**
     * Checks if a new worker can be added with respect to current pool state and the given bound (either core or
     * maximum). If so, the worker count is adjusted accordingly, and, if possible, a new worker is created and started,
     * running firstTask as its first task. This method returns false if the pool is stopped or eligible to shut down.
     * It also returns false if the thread factory fails to create a thread when asked.  If the thread creation fails,
     * either due to the thread factory returning null, or due to an exception (typically OutOfMemoryError in
     * Thread.start()), we roll back cleanly.
     *
     * @param firstTask the task the new thread should run first (or null if none). Workers are created with an initial
     *                  first task (in method execute()) to bypass queuing when there are fewer than corePoolSize
     *                  threads (in which case we always start one), or when the queue is full (in which case we must
     *                  bypass queue). Initially idle threads are usually created via prestartCoreThread or to replace
     *                  other dying workers.
     *                  要跑的任务
     *
     * @param core      if true use corePoolSize as bound, else maximumPoolSize. (A boolean indicator is used here
     *                  rather than a value to ensure reads of fresh values after checking other pool state).
     *                  core=true,表示使用 最小线程数 corePoolSize; core=false,使用最大线程数 maximumPoolSize,
     * @return true if successful true: 表示新任务被接收了,并且执行了. 否则 false
     */
    private boolean addWorker(Runnable firstTask, boolean core) {
        /**
         * 指定的一个 java 标签
         ***示意
         * Java 中的标签是为循环设计的，是为了在多重循环中方便的使用 break 和 coutinue 。
         * 当运行到 break 或 continue 循环时跳到指定的标签处
         */
        retry:
        // 死循环
        for (; ; ) {
            // ctl 是一个 atomic, 高位 0001 xxx
            int c = ctl.get();
            // 获取当前线程状态, 状态存储在 ctl 高位, running:111,shutdown:000,stop:001,tidying:010,terminated:011
            int rs = runStateOf(c);
            
            // Check if queue empty only if necessary.
            /**
             * 这个判断转换成 rs >= SHUTDOWN && (rs != SHUTDOWN || firstTask != null || workQueue.isEmpty)
             * 包括 3 个条件:
             * 1. 线程池不在 running 状态,并且状态是 stop/tidying/terminated 中的任意一种
             * 2. 线程池不在 running 状态,线程池接收了新的任务
             * 3. 线程池不在 running 状态,阻塞队列为空.
             * 满足任意一个,就拒绝执行任务.
             */
            if (rs >= SHUTDOWN &&
                    !(rs == SHUTDOWN &&
                            firstTask == null &&
                            !workQueue.isEmpty())) {
                return false;
            }
            // 死循环
            for (; ; ) {
                // 获取线程池中线程数量
                int wc = workerCountOf(c);
                /**
                 * 如果超出容量或者最大线程池容量,不再接受新任务
                 * CAPACITY 最大数,但是一般不会到达,基本内存会被打爆,所以在使用队列的时候必须指定数量
                 * 那么就是主要判定第二个
                 * wc >= (core ? corePoolSize : maximumPoolSize)
                 * 我们的 demo
                 * wc >= (core ? 2 : 5)
                 * 如果 core = false 说明队列已经满员了，再次进入的任务就会开始创建新的工作线程（如果线程池中的线程数没有到达maximumPoolSize的话）
                 */
                if (wc >= CAPACITY ||
                        wc >= (core ? corePoolSize : maximumPoolSize)) {
                    return false;
                }
                // 使用 cas 操作线程池的线程数 +1 ，cas成功就跳出循环。
                if (compareAndIncrementWorkerCount(c)) {
                    // 跳到 标签，然后执行循环下面的语句，不再进入循环
                    break retry;
                }
                c = ctl.get();  // Re-read ctl
                // 如果线程池状态发生变化，重新循环
                if (runStateOf(c) != rs) {
                    // 跳到标签,继续下一次循环.
                    continue retry;
                }
                // else CAS failed due to workerCount change; retry inner loop
            }
        }
        /**
         * 走到这里说明工作线程数增加成功，线程池线程数量+1
         */
        
        // 任务是否成功启动标识
        boolean workerStarted = false;
        // 任务是否添加成功标识
        boolean workerAdded = false;
        Worker w = null;
        try {
            // 将任务绑定到 worker，之后线程池在调用 worker.run 的时候其实是调用的这个 firstTask.run，我们看 worker 的构造函数有一个指向
            w = new Worker(firstTask);
            // 得到 worker 的 thread 属性，这个thread是我们定义的 ThreadFactory 工厂构建的
            final Thread t = w.thread;
            if (t != null) {
                // 这里得到的是线程池的锁。
                final ReentrantLock mainLock = this.mainLock;
                // 防止并发
                mainLock.lock();
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    int rs = runStateOf(ctl.get());
                    // RUNNING 状态 || SHUTDONW（且任务为 null） 状态下清理队列中剩余的任务
                    if (rs < SHUTDOWN ||
                            (rs == SHUTDOWN && firstTask == null)) {
                        // 判断worker线程是否已经启动了，如果已经启动，就抛出异常
                        if (t.isAlive()) // precheck that t is startable
                        {
                            // 如果存在已经启动并且还没死的线程，抛出异常
                            // worker 是新 new 出来的，thread 就是新构建的，那么线程应该非激活，这里为什么要判断一下呢？
                            throw new IllegalThreadStateException();
                        }
                        // 将新启动的线程添加到线程池中
                        workers.add(w);
                        // 更新线程池线程数且不超过最大值
                        int s = workers.size();
                        if (s > largestPoolSize) {
                            largestPoolSize = s;
                        }
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                // 启动新添加的线程，这个线程首先执行firstTask，然后不停的从队列中取任务执行
                if (workerAdded) {
                    // t 属于 worker，而 thread 又在构建的时候又指向了 worker，所以这个 run 就是调用的 worker 的 run
                    // 我们看代码它调用了 runWorker 方法
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            // 线程启动失败，则从wokers中移除w并递减wokerCount
            if (!workerStarted) {
                addWorkerFailed(w);
            }
        }
        return workerStarted;
    }
    
    /**
     * Rolls back the worker thread creation. - removes worker from workers, if present - decrements worker count -
     * rechecks for termination, in case the existence of this worker was holding up termination
     */
    private void addWorkerFailed(Worker w) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (w != null) {
                workers.remove(w);
            }
            decrementWorkerCount();
            tryTerminate();
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Performs cleanup and bookkeeping for a dying worker. Called only from worker threads. Unless completedAbruptly is
     * set, assumes that workerCount has already been adjusted to account for exit.  This method removes thread from
     * worker set, and possibly terminates the pool or replaces the worker if either it exited due to user task
     * exception or if fewer than corePoolSize workers are running or queue is non-empty but there are no workers.
     *
     * @param w                 the worker
     * @param completedAbruptly if the worker died due to user exception
     */
    private void processWorkerExit(Worker w, boolean completedAbruptly) {
        if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
        {
            decrementWorkerCount();
        }
        
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            completedTaskCount += w.completedTasks;
            workers.remove(w);
        } finally {
            mainLock.unlock();
        }
        
        // 尝试结束线程池的运行。
        tryTerminate();
        
        int c = ctl.get();
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                if (min == 0 && !workQueue.isEmpty()) {
                    min = 1;
                }
                if (workerCountOf(c) >= min) {
                    return; // replacement not needed
                }
            }
            addWorker(null, false);
        }
    }
    
    /**
     * Performs blocking or timed wait for a task, depending on current configuration settings, or returns null if this
     * worker must exit because of any of: 1. There are more than maximumPoolSize workers (due to a call to
     * setMaximumPoolSize). 2. The pool is stopped. 3. The pool is shutdown and the queue is empty. 4. This worker timed
     * out waiting for a task, and timed-out workers are subject to termination (that is, {@code allowCoreThreadTimeOut
     * || workerCount > corePoolSize}) both before and after the timed wait, and if the queue is non-empty, this worker
     * is not the last thread in the pool.
     *
     * worker 被回收的条件
     * 1、worker 数量比线程池限定的 maximumPoolSize 大
     * 2、线程池处于 stop
     * 3、线程池处于 shutdown，且阻塞队列为空
     * 4、使用超时时间从阻塞队列里拿数据，并且超时之后没有拿到数据(allowCoreThreadTimeOut || workerCount > corePoolSize)
     *
     * @return task, or null if the worker must exit, in which case workerCount is decremented
     */
    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?
        
        for (; ; ) {
            int c = ctl.get();
            int rs = runStateOf(c);
            
            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }
            
            int wc = workerCountOf(c);
            
            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
            
            if ((wc > maximumPoolSize || (timed && timedOut))
                    && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c)) {
                    return null;
                }
                continue;
            }
            
            try {
                // 从队列获取 thread
                Runnable r = timed ?
                        workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                        workQueue.take();
                if (r != null) {
                    return r;
                }
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }
    
    /**
     * Main worker run loop.  Repeatedly gets tasks from queue and executes them, while coping with a number of issues:
     * <p>
     * 1. We may start out with an initial task, in which case we don't need to get the first one. Otherwise, as long as
     * pool is running, we get tasks from getTask. If it returns null then the worker exits due to changed pool state or
     * configuration parameters.  Other exits result from exception throws in external code, in which case
     * completedAbruptly holds, which usually leads processWorkerExit to replace this thread.
     * <p>
     * 2. Before running any task, the lock is acquired to prevent other pool interrupts while the task is executing,
     * and then we ensure that unless pool is stopping, this thread does not have its interrupt set.
     * <p>
     * 3. Each task run is preceded by a call to beforeExecute, which might throw an exception, in which case we cause
     * thread to die (breaking loop with completedAbruptly true) without processing the task.
     * <p>
     * 4. Assuming beforeExecute completes normally, we run the task, gathering any of its thrown exceptions to send to
     * afterExecute. We separately handle RuntimeException, Error (both of which the specs guarantee that we trap) and
     * arbitrary Throwables. Because we cannot rethrow Throwables within Runnable.run, we wrap them within Errors on the
     * way out (to the thread's UncaughtExceptionHandler).  Any thrown exception also conservatively causes thread to
     * die.
     * <p>
     * 5. After task.run completes, we call afterExecute, which may also throw an exception, which will also cause
     * thread to die. According to JLS Sec 14.20, this exception is the one that will be in effect even if task.run
     * throws.
     * <p>
     * The net effect of the exception mechanics is that afterExecute and the thread's UncaughtExceptionHandler have as
     * accurate information as we can provide about any problems encountered by user code.
     *
     * @param w the worker
     */
    final void runWorker(Worker w) {
        // 得到当前线程
        Thread wt = Thread.currentThread();
        // 得到要处理的任务 task，【就是用户传入的需要处理的 execute(task)的这个 task】
        Runnable task = w.firstTask;
        // 原有的任务指空
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            /**
             * 因为 worker 会先执行自已已有的任务，然后再去 queue 获取。
             *
             * 1、如果 worker 的任务不为空继续执行
             * 2、否则使用 getTask 方法获得任务。循环直到得到的任务为 null 才退出。
             */
            while (task != null || (task = getTask()) != null) {
                /**
                 * 锁住 worker，表示是有任务的
                 */
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                /**
                 * 1、如果线程池已经处于 stop 状态，且当前线程没有被中断，就去中断
                 * 2、线程池处于 running 或者 shutdown，且当前线程已经被中断，重新检查一下线程池状态，
                 *    如果处于 stop 状态且没中断，就去中断。
                 */
                if ((runStateAtLeast(ctl.get(), STOP) ||
                        (Thread.interrupted() &&
                                runStateAtLeast(ctl.get(), STOP))) &&
                        !wt.isInterrupted()) {
                    wt.interrupt();
                }
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        /**
                         * 真正执行任务，注意是 run 方法，不是 start 方法。
                         * run 方法相当于方法调用，还是同一个线程，但是 start 会启用新线程
                         * 这里的 run 的时候可能被打断，比如线程池调用了 shutdownNew 方法
                         */
                        task.run();
                    } catch (RuntimeException x) {
                        // 异常全部抛出，不再 worker 处理
                        thrown = x;
                        throw x;
                    } catch (Error x) {
                        thrown = x;
                        throw x;
                    } catch (Throwable x) {
                        thrown = x;
                        throw new Error(x);
                    } finally {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    // 记录当前 worker 已经处理的任务数
                    w.completedTasks++;
                    // 释放锁，变为闲置 worker
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            // 回收 worker 的方法，gettask 返回了 null，说明阻塞队列已经没有任务了，那么就需要回收多的 worker
            processWorkerExit(w, completedAbruptly);
        }
    }
    
    // Public constructors and methods
    
    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial parameters and default thread factory and
     * rejected execution handler. It may be more convenient to use one of the {@link Executors} factory methods instead
     * of this general purpose constructor.
     *
     * @param corePoolSize    the number of threads to keep in the pool, even if they are idle, unless {@code
     *                        allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the pool
     * @param keepAliveTime   when the number of threads is greater than the core, this is the maximum time that excess
     *                        idle threads will wait for new tasks before terminating.
     * @param unit            the time unit for the {@code keepAliveTime} argument
     * @param workQueue       the queue to use for holding tasks before they are executed.  This queue will hold only
     *                        the {@code Runnable} tasks submitted by the {@code execute} method.
     * @throws IllegalArgumentException if one of the following holds:<br> {@code corePoolSize < 0}<br> {@code
     *                                  keepAliveTime < 0}<br> {@code maximumPoolSize <= 0}<br> {@code maximumPoolSize <
     *                                  corePoolSize}
     * @throws NullPointerException     if {@code workQueue} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
            int maximumPoolSize,
            long keepAliveTime,
            TimeUnit unit,
            BlockingQueue<Runnable> workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
                Executors.defaultThreadFactory(), defaultHandler);
    }
    
    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial parameters and default rejected execution
     * handler.
     *
     * @param corePoolSize    the number of threads to keep in the pool, even if they are idle, unless {@code
     *                        allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the pool
     * @param keepAliveTime   when the number of threads is greater than the core, this is the maximum time that excess
     *                        idle threads will wait for new tasks before terminating.
     * @param unit            the time unit for the {@code keepAliveTime} argument
     * @param workQueue       the queue to use for holding tasks before they are executed.  This queue will hold only
     *                        the {@code Runnable} tasks submitted by the {@code execute} method.
     * @param threadFactory   the factory to use when the executor creates a new thread
     * @throws IllegalArgumentException if one of the following holds:<br> {@code corePoolSize < 0}<br> {@code
     *                                  keepAliveTime < 0}<br> {@code maximumPoolSize <= 0}<br> {@code maximumPoolSize <
     *                                  corePoolSize}
     * @throws NullPointerException     if {@code workQueue} or {@code threadFactory} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
            int maximumPoolSize,
            long keepAliveTime,
            TimeUnit unit,
            BlockingQueue<Runnable> workQueue,
            ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
                threadFactory, defaultHandler);
    }
    
    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial parameters and default thread factory.
     *
     * @param corePoolSize    the number of threads to keep in the pool, even if they are idle, unless {@code
     *                        allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the pool
     * @param keepAliveTime   when the number of threads is greater than the core, this is the maximum time that excess
     *                        idle threads will wait for new tasks before terminating.
     * @param unit            the time unit for the {@code keepAliveTime} argument
     * @param workQueue       the queue to use for holding tasks before they are executed.  This queue will hold only
     *                        the {@code Runnable} tasks submitted by the {@code execute} method.
     * @param handler         the handler to use when execution is blocked because the thread bounds and queue
     *                        capacities are reached
     * @throws IllegalArgumentException if one of the following holds:<br> {@code corePoolSize < 0}<br> {@code
     *                                  keepAliveTime < 0}<br> {@code maximumPoolSize <= 0}<br> {@code maximumPoolSize <
     *                                  corePoolSize}
     * @throws NullPointerException     if {@code workQueue} or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
            int maximumPoolSize,
            long keepAliveTime,
            TimeUnit unit,
            BlockingQueue<Runnable> workQueue,
            RejectedExecutionHandler handler) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
                Executors.defaultThreadFactory(), handler);
    }
    
    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial parameters.
     *
     * @param corePoolSize    the number of threads to keep in the pool, even if they are idle, unless {@code
     *                        allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the pool
     * @param keepAliveTime   when the number of threads is greater than the core, this is the maximum time that excess
     *                        idle threads will wait for new tasks before terminating.
     * @param unit            the time unit for the {@code keepAliveTime} argument
     * @param workQueue       the queue to use for holding tasks before they are executed.  This queue will hold only
     *                        the {@code Runnable} tasks submitted by the {@code execute} method.
     * @param threadFactory   the factory to use when the executor creates a new thread
     * @param handler         the handler to use when execution is blocked because the thread bounds and queue
     *                        capacities are reached
     * @throws IllegalArgumentException if one of the following holds:<br> {@code corePoolSize < 0}<br> {@code
     *                                  keepAliveTime < 0}<br> {@code maximumPoolSize <= 0}<br> {@code maximumPoolSize <
     *                                  corePoolSize}
     * @throws NullPointerException     if {@code workQueue} or {@code threadFactory} or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
            int maximumPoolSize,
            long keepAliveTime,
            TimeUnit unit,
            BlockingQueue<Runnable> workQueue,
            ThreadFactory threadFactory,
            RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
                maximumPoolSize <= 0 ||
                maximumPoolSize < corePoolSize ||
                keepAliveTime < 0) {
            throw new IllegalArgumentException();
        }
        if (workQueue == null || threadFactory == null || handler == null) {
            throw new NullPointerException();
        }
        this.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }
    
    /**
     * 将传入的 task 在未来的某个时间执行.[线程池不确保任务会立即执行,当然也有可能会立即执行] 新传入的 task 将在一个新的线程或者复用之前线程池已有的线程.
     * 如果传入的任务不被接收,可能是线程池被关闭了,或者达到了容量上限; 那么新提交的任务会交给 RejectedExecutionHandler[拒绝策略] 处理
     * 4 中拒绝策略:
     * 1. 直接报错
     * 2. 忽略新任务不处理,抛弃
     * 3. 将候客厅的[Queue]等待时间最长的任务丢弃,新来的存起来
     * 4. 交给提交任务的线程去自己处理, 比如通过 main 方法提交了任务 executors.execute(task),那么交给 main 线程去处理这个 task
     * Executes the given task sometime in the future.  The task may execute in a new thread or in an existing pooled
     * thread.
     * If the task cannot be submitted for execution, either because this executor has been shutdown or because its
     * capacity has been reached, the task is handled by the current {@code RejectedExecutionHandler}.
     *
     * @param command the task to execute 提交的任务
     * @throws RejectedExecutionException at discretion of {@code RejectedExecutionHandler}, if the task cannot be
     *                                    accepted for execution
     * @throws NullPointerException       if {@code command} is null
     */
    public void execute(Runnable command) {
        // 任务不允许为空
        if (command == null) {
            throw new NullPointerException();
        }
        /*
         * 假设线程池的配置是: new ThreadPoolExecutor(
         * 2,
         * 5,
         * 1000L,
         * TimeUnit.MILLISECONDS,
         * new LinkedBlockingQueue<>(1024),
         * new ThreadFactoryBuilder().setNameFormat("my-pool-%d").build(),
         * new ThreadPoolExecutor.AbortPolicy()
         * )
         *
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 如果当前线程池正在运行的线程数量 workerNum < corePoolSize=2, 那么就创建一个新的 worker 线程用来执行传入的 command/task.
         *
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 判断:如果当前正在执行的 workder 数量大于等于 corePoolSize.
         * 将任务放到阻塞队列 如果阻塞队列没满且 线程池状态为 running,直接放入队列.
         * 否则进行下一步.
         *
         * 放到阻塞队列后,还要再做一次验证
         * (丢入到阻塞队列之后可能另外一个线程关闭了线程池或者刚刚加入到队列的线程死了)
         * 如果这个时候[再次验证]线程池不在 running 状态,把刚刚丢进队列的任务 remove,调用 reject 方法.
         * 否则查看 workder 数量,如果 workder 数量为 0,起一个新的 worker 去阻塞队列里拿任务执行.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         * 如果不能将任务继续存入 queueu 了,就会去尝试开启新的线程,如果开启新线程失败,那么我们就知道线程池要么关闭了,要么就饱和了.
         * 需要拒绝任务, 调用 reject 方法
         *
         */
        int c = ctl.get();
        // 工作线程数 < 核心线程数,
        if (workerCountOf(c) < corePoolSize) {
            // 直接将任务交给 addWorker 处理,然后此方法返回, 第二个参数 true 表示使用 corePoolSize 大小
            if (addWorker(command, true)) {
                return;
            }
            c = ctl.get();
        }
        // workder 数量 > corePoolSize，并且线程池还在RUNNING状态，阻塞队列也没满的情况，加到阻塞队列里
        if (isRunning(c) && workQueue.offer(command)) {
            // 再次检查线程池的运行状态
            int recheck = ctl.get();
            // 虽然满足了第二个步骤，但是这个时候可能突然线程池关闭了，所以再做一层判断
            if (!isRunning(recheck) && remove(command)) {
                reject(command);
            } else if (workerCountOf(recheck) == 0) {
                addWorker(null, false);
            }
        }
        // 直接使用线程池 maximumPoolSize 。addWorker方法第二个参数false表示使用最大 maximumPoolSize
        else if (!addWorker(command, false)) {
            reject(command);
        }
    }
    
    /**
     * Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be
     * accepted. Invocation has no additional effect if already shut down.
     *
     * <p>This method does not wait for previously submitted tasks to
     * complete execution.  Use {@link #awaitTermination awaitTermination} to do that.
     *
     * @throws SecurityException {@inheritDoc}
     */
    public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(SHUTDOWN);
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
    }
    
    /**
     * Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the
     * tasks that were awaiting execution. These tasks are drained (removed) from the task queue upon return from this
     * method.
     *
     * <p>This method does not wait for actively executing tasks to
     * terminate.  Use {@link #awaitTermination awaitTermination} to do that.
     *
     * <p>There are no guarantees beyond best-effort attempts to stop
     * processing actively executing tasks.  This implementation cancels tasks via {@link Thread#interrupt}, so any task
     * that fails to respond to interrupts may never terminate.
     *
     * @throws SecurityException {@inheritDoc}
     */
    public List<Runnable> shutdownNow() {
        List<Runnable> tasks;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(STOP);
            interruptWorkers();
            tasks = drainQueue();
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
        return tasks;
    }
    
    public boolean isShutdown() {
        return !isRunning(ctl.get());
    }
    
    /**
     * Returns true if this executor is in the process of terminating after {@link #shutdown} or {@link #shutdownNow}
     * but has not completely terminated.  This method may be useful for debugging. A return of {@code true} reported a
     * sufficient period after shutdown may indicate that submitted tasks have ignored or suppressed interruption,
     * causing this executor not to properly terminate.
     *
     * @return {@code true} if terminating but not yet terminated
     */
    public boolean isTerminating() {
        int c = ctl.get();
        return !isRunning(c) && runStateLessThan(c, TERMINATED);
    }
    
    public boolean isTerminated() {
        return runStateAtLeast(ctl.get(), TERMINATED);
    }
    
    public boolean awaitTermination(long timeout, TimeUnit unit)
            throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (; ; ) {
                if (runStateAtLeast(ctl.get(), TERMINATED)) {
                    return true;
                }
                if (nanos <= 0) {
                    return false;
                }
                nanos = termination.awaitNanos(nanos);
            }
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Invokes {@code shutdown} when this executor is no longer referenced and it has no threads.
     */
    protected void finalize() {
        SecurityManager sm = System.getSecurityManager();
        if (sm == null || acc == null) {
            shutdown();
        } else {
            PrivilegedAction<Void> pa = () -> {
                shutdown();
                return null;
            };
            AccessController.doPrivileged(pa, acc);
        }
    }
    
    /**
     * Sets the thread factory used to create new threads.
     *
     * @param threadFactory the new thread factory
     * @throws NullPointerException if threadFactory is null
     * @see #getThreadFactory
     */
    public void setThreadFactory(ThreadFactory threadFactory) {
        if (threadFactory == null) {
            throw new NullPointerException();
        }
        this.threadFactory = threadFactory;
    }
    
    /**
     * Returns the thread factory used to create new threads.
     *
     * @return the current thread factory
     * @see #setThreadFactory(ThreadFactory)
     */
    public ThreadFactory getThreadFactory() {
        return threadFactory;
    }
    
    /**
     * Sets a new handler for unexecutable tasks.
     *
     * @param handler the new handler
     * @throws NullPointerException if handler is null
     * @see #getRejectedExecutionHandler
     */
    public void setRejectedExecutionHandler(RejectedExecutionHandler handler) {
        if (handler == null) {
            throw new NullPointerException();
        }
        this.handler = handler;
    }
    
    /**
     * Returns the current handler for unexecutable tasks.
     *
     * @return the current handler
     * @see #setRejectedExecutionHandler(RejectedExecutionHandler)
     */
    public RejectedExecutionHandler getRejectedExecutionHandler() {
        return handler;
    }
    
    /**
     * Sets the core number of threads.  This overrides any value set in the constructor.  If the new value is smaller
     * than the current value, excess existing threads will be terminated when they next become idle.  If larger, new
     * threads will, if needed, be started to execute any queued tasks.
     *
     * @param corePoolSize the new core size
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     * @see #getCorePoolSize
     */
    public void setCorePoolSize(int corePoolSize) {
        if (corePoolSize < 0) {
            throw new IllegalArgumentException();
        }
        int delta = corePoolSize - this.corePoolSize;
        this.corePoolSize = corePoolSize;
        if (workerCountOf(ctl.get()) > corePoolSize) {
            interruptIdleWorkers();
        } else if (delta > 0) {
            // We don't really know how many new threads are "needed".
            // As a heuristic, prestart enough new workers (up to new
            // core size) to handle the current number of tasks in
            // queue, but stop if queue becomes empty while doing so.
            int k = Math.min(delta, workQueue.size());
            while (k-- > 0 && addWorker(null, true)) {
                if (workQueue.isEmpty()) {
                    break;
                }
            }
        }
    }
    
    /**
     * Returns the core number of threads.
     *
     * @return the core number of threads
     * @see #setCorePoolSize
     */
    public int getCorePoolSize() {
        return corePoolSize;
    }
    
    /**
     * Starts a core thread, causing it to idly wait for work. This overrides the default policy of starting core
     * threads only when new tasks are executed. This method will return {@code false} if all core threads have already
     * been started.
     *
     * @return {@code true} if a thread was started
     */
    public boolean prestartCoreThread() {
        return workerCountOf(ctl.get()) < corePoolSize &&
                addWorker(null, true);
    }
    
    /**
     * Same as prestartCoreThread except arranges that at least one thread is started even if corePoolSize is 0.
     */
    void ensurePrestart() {
        int wc = workerCountOf(ctl.get());
        if (wc < corePoolSize) {
            addWorker(null, true);
        } else if (wc == 0) {
            addWorker(null, false);
        }
    }
    
    /**
     * Starts all core threads, causing them to idly wait for work. This overrides the default policy of starting core
     * threads only when new tasks are executed.
     *
     * @return the number of threads started
     */
    public int prestartAllCoreThreads() {
        int n = 0;
        while (addWorker(null, true)) {
            ++n;
        }
        return n;
    }
    
    /**
     * Returns true if this pool allows core threads to time out and terminate if no tasks arrive within the keepAlive
     * time, being replaced if needed when new tasks arrive. When true, the same keep-alive policy applying to non-core
     * threads applies also to core threads. When false (the default), core threads are never terminated due to lack of
     * incoming tasks.
     *
     * @return {@code true} if core threads are allowed to time out, else {@code false}
     * @since 1.6
     */
    public boolean allowsCoreThreadTimeOut() {
        return allowCoreThreadTimeOut;
    }
    
    /**
     * Sets the policy governing whether core threads may time out and terminate if no tasks arrive within the
     * keep-alive time, being replaced if needed when new tasks arrive. When false, core threads are never terminated
     * due to lack of incoming tasks. When true, the same keep-alive policy applying to non-core threads applies also to
     * core threads. To avoid continual thread replacement, the keep-alive time must be greater than zero when setting
     * {@code true}. This method should in general be called before the pool is actively used.
     *
     * @param value {@code true} if should time out, else {@code false}
     * @throws IllegalArgumentException if value is {@code true} and the current keep-alive time is not greater than
     *                                  zero
     * @since 1.6
     */
    public void allowCoreThreadTimeOut(boolean value) {
        if (value && keepAliveTime <= 0) {
            throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
        }
        if (value != allowCoreThreadTimeOut) {
            allowCoreThreadTimeOut = value;
            if (value) {
                interruptIdleWorkers();
            }
        }
    }
    
    /**
     * Sets the maximum allowed number of threads. This overrides any value set in the constructor. If the new value is
     * smaller than the current value, excess existing threads will be terminated when they next become idle.
     *
     * @param maximumPoolSize the new maximum
     * @throws IllegalArgumentException if the new maximum is less than or equal to zero, or less than the {@linkplain
     *                                  #getCorePoolSize core pool size}
     * @see #getMaximumPoolSize
     */
    public void setMaximumPoolSize(int maximumPoolSize) {
        if (maximumPoolSize <= 0 || maximumPoolSize < corePoolSize) {
            throw new IllegalArgumentException();
        }
        this.maximumPoolSize = maximumPoolSize;
        if (workerCountOf(ctl.get()) > maximumPoolSize) {
            interruptIdleWorkers();
        }
    }
    
    /**
     * Returns the maximum allowed number of threads.
     *
     * @return the maximum allowed number of threads
     * @see #setMaximumPoolSize
     */
    public int getMaximumPoolSize() {
        return maximumPoolSize;
    }
    
    /**
     * Sets the time limit for which threads may remain idle before being terminated.  If there are more than the core
     * number of threads currently in the pool, after waiting this amount of time without processing a task, excess
     * threads will be terminated.  This overrides any value set in the constructor.
     *
     * @param time the time to wait.  A time value of zero will cause excess threads to terminate immediately after
     *             executing tasks.
     * @param unit the time unit of the {@code time} argument
     * @throws IllegalArgumentException if {@code time} less than zero or if {@code time} is zero and {@code
     *                                  allowsCoreThreadTimeOut}
     * @see #getKeepAliveTime(TimeUnit)
     */
    public void setKeepAliveTime(long time, TimeUnit unit) {
        if (time < 0) {
            throw new IllegalArgumentException();
        }
        if (time == 0 && allowsCoreThreadTimeOut()) {
            throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
        }
        long keepAliveTime = unit.toNanos(time);
        long delta = keepAliveTime - this.keepAliveTime;
        this.keepAliveTime = keepAliveTime;
        if (delta < 0) {
            interruptIdleWorkers();
        }
    }
    
    /**
     * Returns the thread keep-alive time, which is the amount of time that threads in excess of the core pool size may
     * remain idle before being terminated.
     *
     * @param unit the desired time unit of the result
     * @return the time limit
     * @see #setKeepAliveTime(long, TimeUnit)
     */
    public long getKeepAliveTime(TimeUnit unit) {
        return unit.convert(keepAliveTime, TimeUnit.NANOSECONDS);
    }
    
    /* User-level queue utilities */
    
    /**
     * Returns the task queue used by this executor. Access to the task queue is intended primarily for debugging and
     * monitoring. This queue may be in active use.  Retrieving the task queue does not prevent queued tasks from
     * executing.
     *
     * @return the task queue
     */
    public BlockingQueue<Runnable> getQueue() {
        return workQueue;
    }
    
    /**
     * Removes this task from the executor's internal queue if it is present, thus causing it not to be run if it has
     * not already started.
     *
     * <p>This method may be useful as one part of a cancellation
     * scheme.  It may fail to remove tasks that have been converted into other forms before being placed on the
     * internal queue. For example, a task entered using {@code submit} might be converted into a form that maintains
     * {@code Future} status. However, in such cases, method {@link #purge} may be used to remove those Futures that
     * have been cancelled.
     *
     * @param task the task to remove
     * @return {@code true} if the task was removed
     */
    public boolean remove(Runnable task) {
        boolean removed = workQueue.remove(task);
        tryTerminate(); // In case SHUTDOWN and now empty
        return removed;
    }
    
    /**
     * Tries to remove from the work queue all {@link Future} tasks that have been cancelled. This method can be useful
     * as a storage reclamation operation, that has no other impact on functionality. Cancelled tasks are never
     * executed, but may accumulate in work queues until worker threads can actively remove them. Invoking this method
     * instead tries to remove them now. However, this method may fail to remove tasks in the presence of interference
     * by other threads.
     */
    public void purge() {
        final BlockingQueue<Runnable> q = workQueue;
        try {
            Iterator<Runnable> it = q.iterator();
            while (it.hasNext()) {
                Runnable r = it.next();
                if (r instanceof Future<?> && ((Future<?>) r).isCancelled()) {
                    it.remove();
                }
            }
        } catch (ConcurrentModificationException fallThrough) {
            // Take slow path if we encounter interference during traversal.
            // Make copy for traversal and call remove for cancelled entries.
            // The slow path is more likely to be O(N*N).
            for (Object r : q.toArray()) {
                if (r instanceof Future<?> && ((Future<?>) r).isCancelled()) {
                    q.remove(r);
                }
            }
        }
        
        tryTerminate(); // In case SHUTDOWN and now empty
    }
    
    /* Statistics */
    
    /**
     * Returns the current number of threads in the pool.
     *
     * @return the number of threads
     */
    public int getPoolSize() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // Remove rare and surprising possibility of
            // isTerminated() && getPoolSize() > 0
            return runStateAtLeast(ctl.get(), TIDYING) ? 0
                    : workers.size();
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Returns the approximate number of threads that are actively executing tasks.
     *
     * @return the number of threads
     */
    public int getActiveCount() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            int n = 0;
            for (Worker w : workers) {
                if (w.isLocked()) {
                    ++n;
                }
            }
            return n;
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Returns the largest number of threads that have ever simultaneously been in the pool.
     *
     * @return the number of threads
     */
    public int getLargestPoolSize() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            return largestPoolSize;
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Returns the approximate total number of tasks that have ever been scheduled for execution. Because the states of
     * tasks and threads may change dynamically during computation, the returned value is only an approximation.
     *
     * @return the number of tasks
     */
    public long getTaskCount() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            long n = completedTaskCount;
            for (Worker w : workers) {
                n += w.completedTasks;
                if (w.isLocked()) {
                    ++n;
                }
            }
            return n + workQueue.size();
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Returns the approximate total number of tasks that have completed execution. Because the states of tasks and
     * threads may change dynamically during computation, the returned value is only an approximation, but one that does
     * not ever decrease across successive calls.
     *
     * @return the number of tasks
     */
    public long getCompletedTaskCount() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            long n = completedTaskCount;
            for (Worker w : workers) {
                n += w.completedTasks;
            }
            return n;
        } finally {
            mainLock.unlock();
        }
    }
    
    /**
     * Returns a string identifying this pool, as well as its state, including indications of run state and estimated
     * worker and task counts.
     *
     * @return a string identifying this pool, as well as its state
     */
    public String toString() {
        long ncompleted;
        int nworkers, nactive;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            ncompleted = completedTaskCount;
            nactive = 0;
            nworkers = workers.size();
            for (Worker w : workers) {
                ncompleted += w.completedTasks;
                if (w.isLocked()) {
                    ++nactive;
                }
            }
        } finally {
            mainLock.unlock();
        }
        int c = ctl.get();
        String rs = (runStateLessThan(c, SHUTDOWN) ? "Running" :
                (runStateAtLeast(c, TERMINATED) ? "Terminated" :
                        "Shutting down"));
        return super.toString() +
                "[" + rs +
                ", pool size = " + nworkers +
                ", active threads = " + nactive +
                ", queued tasks = " + workQueue.size() +
                ", completed tasks = " + ncompleted +
                "]";
    }
    
    /* Extension hooks */
    
    /**
     * Method invoked prior to executing the given Runnable in the given thread.  This method is invoked by thread
     * {@code t} that will execute task {@code r}, and may be used to re-initialize ThreadLocals, or to perform
     * logging.
     *
     * <p>This implementation does nothing, but may be customized in
     * subclasses. Note: To properly nest multiple overridings, subclasses should generally invoke {@code
     * super.beforeExecute} at the end of this method.
     *
     * @param t the thread that will run task {@code r}
     * @param r the task that will be executed
     */
    protected void beforeExecute(Thread t, Runnable r) {
    }
    
    /**
     * Method invoked upon completion of execution of the given Runnable. This method is invoked by the thread that
     * executed the task. If non-null, the Throwable is the uncaught {@code RuntimeException} or {@code Error} that
     * caused execution to terminate abruptly.
     *
     * <p>This implementation does nothing, but may be customized in
     * subclasses. Note: To properly nest multiple overridings, subclasses should generally invoke {@code
     * super.afterExecute} at the beginning of this method.
     *
     * <p><b>Note:</b> When actions are enclosed in tasks (such as
     * {@link FutureTask}) either explicitly or via methods such as {@code submit}, these task objects catch and
     * maintain computational exceptions, and so they do not cause abrupt termination, and the internal exceptions are
     * <em>not</em> passed to this method. If you would like to trap both kinds of failures in this method, you can
     * further probe for such cases, as in this sample subclass that prints either the direct cause or the underlying
     * exception if a task has been aborted:
     *
     * <pre> {@code
     * class ExtendedExecutor extends ThreadPoolExecutor {
     *   // ...
     *   protected void afterExecute(Runnable r, Throwable t) {
     *     super.afterExecute(r, t);
     *     if (t == null && r instanceof Future<?>) {
     *       try {
     *         Object result = ((Future<?>) r).get();
     *       } catch (CancellationException ce) {
     *           t = ce;
     *       } catch (ExecutionException ee) {
     *           t = ee.getCause();
     *       } catch (InterruptedException ie) {
     *           Thread.currentThread().interrupt(); // ignore/reset
     *       }
     *     }
     *     if (t != null)
     *       System.out.println(t);
     *   }
     * }}</pre>
     *
     * @param r the runnable that has completed
     * @param t the exception that caused termination, or null if execution completed normally
     */
    protected void afterExecute(Runnable r, Throwable t) {
    }
    
    /**
     * Method invoked when the Executor has terminated.  Default implementation does nothing. Note: To properly nest
     * multiple overridings, subclasses should generally invoke {@code super.terminated} within this method.
     */
    protected void terminated() {
    }
    
    /* Predefined RejectedExecutionHandlers */
    
    /**
     * A handler for rejected tasks that runs the rejected task directly in the calling thread of the {@code execute}
     * method, unless the executor has been shut down, in which case the task is discarded.
     */
    public static class CallerRunsPolicy implements RejectedExecutionHandler {
        
        /**
         * Creates a {@code CallerRunsPolicy}.
         */
        public CallerRunsPolicy() {
        }
        
        /**
         * Executes task r in the caller's thread, unless the executor has been shut down, in which case the task is
         * discarded.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                r.run();
            }
        }
    }
    
    /**
     * A handler for rejected tasks that throws a {@code RejectedExecutionException}.
     */
    public static class AbortPolicy implements RejectedExecutionHandler {
        
        /**
         * Creates an {@code AbortPolicy}.
         */
        public AbortPolicy() {
        }
        
        /**
         * Always throws RejectedExecutionException.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         * @throws RejectedExecutionException always
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException("Task " + r.toString() +
                    " rejected from " +
                    e.toString());
        }
    }
    
    /**
     * A handler for rejected tasks that silently discards the rejected task.
     */
    public static class DiscardPolicy implements RejectedExecutionHandler {
        
        /**
         * Creates a {@code DiscardPolicy}.
         */
        public DiscardPolicy() {
        }
        
        /**
         * Does nothing, which has the effect of discarding task r.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        }
    }
    
    /**
     * A handler for rejected tasks that discards the oldest unhandled request and then retries {@code execute}, unless
     * the executor is shut down, in which case the task is discarded.
     */
    public static class DiscardOldestPolicy implements RejectedExecutionHandler {
        
        /**
         * Creates a {@code DiscardOldestPolicy} for the given executor.
         */
        public DiscardOldestPolicy() {
        }
        
        /**
         * Obtains and ignores the next task that the executor would otherwise execute, if one is immediately available,
         * and then retries execution of task r, unless the executor is shut down, in which case task r is instead
         * discarded.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                e.getQueue().poll();
                e.execute(r);
            }
        }
    }
}
